Bioreactors are used in the bioprocess industry for growing cells, e.g. for the purpose of producing biopharmaceuticals. Disposable bioreactors have been used more and more and are an increasing trend in the industry. These disposable bioreactors are flexible bags made of plastic, for example welded polyethylene film. For the culturing of cells it is important to measure some features of the culture inside the bioreactor. The features could be for example temperature, pH, and dissolved oxygen (DO) etc. In flexible bags it is common to provide sensors from the upper side of the bag, like the Cellbag™ 50 L/pH from GE Healthcare. One disadvantage with this is the risk of the sensor not being inside the culture all the time. Optical sensors for measuring for example pH and DO are provided from for example PreSens. Such an optical sensor, sometimes called a sensor spot, is a spot of a dye material typically comprising two different fluorescent materials. When the sensor spot is illuminated, fluorescent substances of the spot will emit light. Using suitable fluorescent substances this emitted light can be indicative of a property of the culture, such as pH or DO. Thus the returned light can be analysed and for example pH or DO can be determined. For optimal performance, these sensors should be provided in the culture all the time. Providing the sensors in the bottom part of the bag is often not as convenient as providing the sensor from the upper side due to the support structure often provided under the bag. Furthermore these optical sensors typically can not be attached directly on the material of the bag due to chemical incompatibility. Another issue is that the sensor assembly should be as flat as possible to ensure that it always will remain under the liquid surface.